The present invention relates to a semiconductor laser and more particularly to a heterojunction semiconductor laser having a structure that is effective for controlling an oscillation mode.
In order to bring a semiconductor laser into continuous oscillation at a high temperature, it is necessary to employ such structure that the best thermal path for removing heat from the junction in the semiconductor laser can be provided and that an optical energy and an injection current can be confined in a particular region where an optical loss and wasteful carrier recombinations are simultaneously minimized.
To meet this condition, the so-called contact stripe-geometry semiconductor laser has been proposed, in which the electrode of the semiconductor laser is formed in a stripe shape, so that the width of the current flowing into the active layer is limited and the optical energy is confined in the active layer.
However, despite of the advantage of this semiconductor laser that oscillation by a direct current at a room temperature has become possible, it has a great disadvantage in performance in that an electromagnetic wave mode standing in the transversal direction parallel to the active layer, i.e., a transverse mode, is unstable and the transverse mode varies in response to variation of the injection current. This is due to the fact that the contact stripe-geometry laser does not have the function of confining a carrier and light in the active layer with respect to the transverse direction thereof. More particularly, in the current region slightly above the starting current value for the laser oscillation, the gain necessary for oscillation exceeds the loss only in the active layer region right under the stripe, and hence, the laser oscillates in a low or zeroth order of transverse mode. However, as the injection current is increased, the carriers injected into the active layer spread toward outside regions, so that the high gain region expands, resulting in a spreading of a transverse mode and generation of higher order modes. The instability and injection current dependency of the transverse mode becomes a cause of mode dispersion and the like in an optical transmission path in the case of carrying out an optical fiber communication by means of laser light, and thus extremely lowers the information capacity of the transmission path. Accordingly, a semiconductor laser used as an optical source in an optical fiber communication system is required to oscillate in a single mode over a large injection current region. Therefore, trials have been made so as to obviate such disadvantage by structurally incorporating waveguide means within the laser element. For instance, the strip buried heterostructure (SBH) laser proposed by W. T. Tsang et al on pages 311 to 314 of Applied Physics Letters Vol. 32, No. 5, Mar. 1, 1978 is understood to be one of such trials. In this structure, as will be later described more in detail, a waveguide layer is provided in addition to the active layer, and only the active layer is surrounded by a substance having a low refractive index, so that the confining of injected carriers and the confining of photons are respectively effected in separate regions and a light propagation effect is provided by the waveguide layer. Thus, it is intended to prevent higher-order oscillations and to maintain a single mode oscillation over a large current region.
However, the manufacture of the SBH lasers has an obvious shortcoming in that it necessitates different individual processes of epitaxial growth. Accordingly, such SBH lasers involve the problem that the process of manufacture is so complex that reproducibility in the manufacture of the laser elements is poor and they are not suitable for realizing economy and mass-producibility.
On the other hand, as disclosed in U.S. Pat. No. 3,978,428 issued to R. D. Burnham et al, the etched buried heterostructure (EBH) laser has been known, in which an output beam configuration and stability of a transverse mode is improved by providing a groove in a substrate to curve an active layer. However, in this structure, since the active layer is directly sandwiched by cladding layers having a low refractive index, the difference in the refractive index becomes large at these interfaces. Consequently, if the width of the curved section of the active layer is selected to be large, then higher-order transverse modes are liable to oscillate. From the view point of ease of the crystal growth, the manufacture is easier when the width of the groove in the substrate is broad to a certain extent than when it is narrow. However, at the groove width necessitated for oscillation in a fundamental transverse mode, such laser structure has a disadvantage in that controllability and reproducibility are poor.